This invention relates to the general field of compliant foil fluid film bearings and more particularly to an improved thrust bearing employing fluid foils and multiple spring foils to support, position, damp and accommodate movements or excursions of the rotating portion of the bearing.
Compliant foil fluid film thrust bearings are currently being utilized in a variety of high speed rotor applications. These bearings are generally comprised of a two sided thrust disk rotating element, non-rotating compliant fluid foil members that axially enclose the rotating element, non-rotating compliant spring foil members that axially enclose the fluid foil members and a non-rotating thrust plate element and a non-rotating housing element that axially enclose and provide attachments for the foil members. The space between the rotating element and the thrust plate element on one side of the bearing and the space between the rotating element and the thrust surface of the housing element on the other side of the bearing are filled with fluid (usually air) which envelops the foils.
The rotary motion of the rotating element applies viscous drag forces to the fluid and induces circumferential flow of the fluid between the smooth surface of the rotating element and the fluid foil. The space between the rotating element and the fluid foil is subdivided into a plurality of fluid-dynamic wedge channels. These wedge channels have typically been formed by resistance welding compliant, convex curved foil pads to an underlying support foil. The leading ramps of the foil pads relative to the fluid""s circumferential flow and the smooth surface of the rotating element form the two primary surfaces of the converging wedge channels. The trailing ramps and the smooth surface of the rotating element form the primary surfaces of the diverging wedge channels. The fluid flowing circumferentially along a converging wedge channel experiences steadily decreasing flow area, increasing circumferential flow velocity and increasing static fluid pressure. If the rotating element moves toward the non-rotating element, the convergence angle of the wedge channel increases causing the fluid pressure rise along the channel to increase. If the rotating element moves away, the pressure rise along the wedge channel decreases. Thus, the fluid in the wedge channels exerts restoring forces on the rotating element that vary with and stabilize running clearances and prevent contact between the rotating and non-rotating elements of the bearing. Flexing and sliding of the foils causes coulomb damping of any axial or overturning motion of the rotating element of the bearing.
Owing to preload spring forces or gravity forces, the rotating element of the bearing is typically in physical contact with the fluid foil members of the bearing at low rotational speeds. This physical contact results in bearing wear. It is only when the rotor speed is above what is termed the lift-off/touch-down speed that the fluid dynamic forces generated in the wedge channels assure a running gap between the rotating and non-rotating elements.
Conventional, compliant foil fluid film thrust bearings have fluid dynamic wedge channel ramps that converge or diverge circumferentially with no radial component to the ramp slopes. The converging wedge channel ramps have no side wall or other constraints to prevent fluid flow out of the channels at their inner and outer edges. At the trailing edge of the converging wedge channel, the high fluid pressure and lack of radial flow constraints induces radial flow leakage out of the channel, which in turn, results in a reduction in fluid pressure, a loss in bearing load capacity, and an increase in bearing drag. The radial flow leakage requires make-up flow at the beginning of the converging wedge channel.
Conventional, compliant foil fluid film thrust bearings have primary fluid flow patterns in the converging wedge channels that are single path recirculating loops. The fluid in the converging wedge channels adjacent to the rotating disk travels circumferentially in the same direction as the disk""s motion (up the ramp) owing to viscous drag. The fluid in the converging wedge channels adjacent to the non-rotating fluid foil travels-circumferentially in the direction opposite to the disk""s motion (down the ramp) owing to the circumferential pressure gradient along the channel. Much of the fluid that travels up the ramp near the disk while gaining static pressure turns back before reaching the end of the wedge channel and travels down the ramp near the fluid foil while losing pressure. Almost all of this fluid turns again before reaching the beginning of the wedge channels and travels up the ramp while again gaining pressure. The fluid traveling the single path recirculating loop flow patterns travels essentially the same path each loop and experiences the same pressure increases and pressure decreases each loop with no net pressure gain from one loop to the next. These bearings generate less fluid dynamic pressure and have less load capacity than bearings that utilize multi-path vortex flow patterns where the flow traveling each regenerative loop travels a different path and where there is a net increase in fluid pressure each loop
Conventional, compliant foil fluid film thrust bearings operate with extremely small running clearances and moderate as opposed to low drag and power consumption. The clearances between the non-rotating fluid foil""s converging channel ramp trailing ends and the rotating thrust disk are typically less than 100 micro-inches when the bearing is heavily loaded at operating conditions. The bearing""s drag coefficient is typically more than 0.005 at operating speed as defined by the ratio of the fluid dynamic drag induced shear forces applied to the disk by the bearing divided by the thrust load carried by the bearing.
Compliant foil fluid film thrust bearings tend to rely on backing springs to preload the fluid foils against the relatively moveable rotating element (thrust disk) so as to control foil position/nesting and to establish foil dynamic stability. The bearing starting torque (which should ideally be zero) is directly proportional to these preload forces. These preload forces also significantly increase the disk speed at which the hydrodynamic effects in the wedge channels are strong enough to lift the rotating element of the bearing out of physical contact with the non-rotating members of the bearing. These preload forces and the high lift-off/touch-down speeds result in significant bearing wear each time the disk is started or stopped.
Many conventional, compliant foil fluid film thrust bearings have large sway spaces and loose compliance, i.e. they do not tightly restrict the axial or overturning motion of the bearing thrust disk, owing to poor control of spring deflection tolerances inherent in the spring designs.
It has been common for compliant foil fluid film thrust bearings to utilize a plurality of coated, convex curved, compliant fluid foil pads that are welded to a support foil to form the fluid foil member of the bearing. These two piece fluid foil members are typically thicker and have poorer thickness control than can single piece fluid foil members. Two piece fluid foil members also experience process fluid foil turbulence, increased drag at operating speeds and reduced load capacity owing to the flow discontinuities between the trailing edges of each foil pad and the weld attachment edge of the next circumferentially located pad.
Some conventional, compliant foil fluid film thrust bearings utilize spring foil elements that are formed by milling (chemically or otherwise) circumferentially offset recesses in opposing sides of flat foil stock so as to leave circumferentially offset unmilled ridges on opposing sides of the foil elements. Pressure applied to the offset ridges induces the spring foil element to deflect in a spring-like manner. Spring foil elements formed in this manner are prone to large variations in their spring rates due to small variations in milling depth. This milling process non-symetrically relieves any residual surface compressive stresses induced by previous foil rolling operations and thus induces foil warpage.
Other bearings utilize convolute shaped spring foil elements that are formed by pressing annealed Inconel 750X foil blanks between two contoured plates having matching wavy contours with constant plate to plate spacing and then heat treating the foil blanks at approximately 1300 degrees Fahrenheit for approximately 20 hours while they are still pressed between the contoured plates. Spring foils formed in this manner are prone to have large variations in undeflected thickness.
In some cases, the fluid foils may be attached to the spring foils by welding or brazing or various spring foil elements may be welded or brazed together to form a spring foil member. Those thrust bearings that utilize welding or brazing to attach one foil element to another are subject to foil distortions and foil fatigue failures, particularly at the bond sites.
The sides of the fluid foils that face the rotating element of the bearing can utilize low rubbing friction coatings to minimize bearing wear when disk speed is below the lift-off/touch-down speed. These coatings, however, may have large thickness tolerances that can adversely affect the foil pack thickness tolerance.
The latest development in compliant foil fluid film thrust bearings, described in U.S. Pat. No. 5,529,398 issued Jun. 25, 1996 to Robert W. Bosley entitled xe2x80x9cCompliant Foil Hydrodynamic Fluid Film Thrust Bearingxe2x80x9d includes a self shimming capability to compensate for variations in foil pack thickness and three (3) spring or support foils beneath the fluid foil.
In the present invention, the compliant foil fluid film thrust bearing generally comprises a single sided or two sided thrust disk rotor, fluid foils, spring /s, a thrust plate, a foil retaining housing and a spacer ring. The non-rotating but compliant fluid foils are located adjacent to the thrust face or faces of the rotatable disk. The fluid foils have open faced channels that induce regenerative vortex flow patterns in the process fluid. The forces applied by the thrust disk to the fluid foils vary inversely with fluid foil to disk gap and vary proportionally with disk deflection.
The spring foils provide a tilting pad support for the fluid foils but allow them to follow the axial and overturning motion of the disk. Each of the types of foils, namely fluid foils and spring foils are attached to the foil retaining housing by a compliant web structure and pins. The foils are formed as thin, flat, annular sheets with integral shim rings at their periphery and contoured cutout patterns that are unique to each type of foil.
As part of the forming process, the fluid foil blank is coated on one side with a compliant, wear resistant material then stamped with a forming tool to form the fluid flow channels. The thrust plate is preloaded towards the thrust surface of the foil retaining housing by a preload spring and is held away from the housing by the total thickness of the foil shim rings and the thickness of the spacer ring. This allows the bearing to essentially self shim itself to establish a small clearance between the fluid foils and the disk that is not affected by normal variations in foil or foil coating thicknesses.
The bearing has no preload force and has zero starting torque when the rotor""s axis of rotation is oriented ninety degrees to the force of gravity. Owing to the vortex flow pattern of the process fluid, the bearing running clearances and load capacities are improved while lift-off speeds are reduced. In addition, good damping, low running torque and small sway space are achieved. This is all accomplished at a low manufacturing cost with a low parts count.
It is, therefore, a principal object of the present invention to provide an improved compliant foil fluid film thrust bearing.
It is another object of the present invention to provide such a bearing with enhanced axial and overturning load carrying capacity.
It is another object of the present invention to provide such a bearing with both squeeze film and coulomb damping.
It is another object of the present invention to provide such a bearing with small sway space clearances to tightly restrict bearing and thrust disk rotor deflections.
It is another object of the present invention to provide such a bearing with very low operating torque.
It is another object of the present invention to provide such a bearing with large running clearances between the fluid foil elements and the thrust disk.
It is another object of the present invention to provide such a bearing with fluid foil members that are not preloaded by spring forces against the thrust disk at zero speed.
It is another object of the present invention to provide such a bearing with zero starting torque when there is no gravity induced preload forces.
It is another object of the present invention to provide such a bearing with an extremely low lift-off/touch-down speed which is consistent with zero preload forces.
It is another object of the present invention to provide such a bearing with very low starting and stopping wear which is consistent with zero preload forces and a low lift-off/touch-down speed.
It is another object of the present invention to provide such a bearing with converging wedge channel features (formed on the surface of the fluid foil element) that limit fluid foil losses from the channel at the radial outer and radial inner edges of those channels.
It is another object of the present invention to provide such a bearing with converging wedge channel ramps formed on the surface of the fluid foil elements that have compound curve profiles with concave curvatures radially, flat slopes circumferentially at zero speed and convex curvatures at operating speed when fluid dynamic and spring forces are applied to the fluid foil elements. The profiles will form and function as scoops with radially wide fluid foil inlets, radially narrowing channel widths along the circumferential fluid foil paths, and rounded circumferentially trailing edges.
It is another object of the present invention to provide such a bearing with a fluid foil pattern that reduces fluid pressure losses when the process fluid travels xe2x80x9cdown the rampxe2x80x9d(in a nominally circumferential direction that is opposite to the rotation of the thrust disk) adjacent to the fluid foil element.
It is another object of the present invention to provide such a bearing with a fluid flow pattern that is regenerative with a different flow path for each regenerative flow loop.
It is another object of the present invention to provide such a bearing with a vortex flow pattern.
It is another object of the present invention to provide such a bearing with fluid flow element blanks and spring foil elements that are fabricated by optically masked chemical etch techniques.
It is another object of the present invention to provide such a bearing with foil elements that are extremely flat owing to the processes used to roll and heat treat the foil metal and the processes used to form (e.g. etch) the foil blanks and elements.
It is another object of the present invention to provide such a bearing with foil elements that have tightly held thickness tolerances.
It is another object of the present invention to provide such a bearing with fluid foil members that are single fluid foil elements, one for each side of the bearing.
It is another object of the present invention to provide such a bearing with fluid foil elements that are formed from blanks by pressing steeply sloped joggles to function as diverging wedge channels which allowing the gradually converging wedge channel ramps to result without plastic deformation as the straight line connection between the joggles.
It is another object of the present invention to provide such a bearing with fluid foil elements that are formed from annealed blanks of nickel steel, such as Inconel 750X, by pressing at room temperature.
It is another object of the present invention to provide such a bearing with a spring foil member that has local spring rates that vary with radial and circumferential location so as to accommodate variations in fluid pressure within the converging wedge channel adjacent to the local areas of the spring foil member.
It is another object of the present invention to provide such a bearing with a tilting pad spring support system that controls the relative support forces applied to the underside of the fluid foil at a multiplicity of locations circumferentially along the converging wedge channel.
It is another object of the present invention to provide such a bearing with a tilting pad spring support system having circumferentially closer spacing between support/pivot lines moving up the converging ramp from the leading edge of the fluid foil to the trailing edge of the fluid foil.
It is another object of the present invention to provide such a bearing with a tilting pad spring support system providing increasing support forces and support spring rates moving up the converging ramp from the leading edge of the fluid foil to the trailing edge of the fluid foil.
It is another object of the present invention to provide such a bearing with a tilting pad spring support system with radially curved support/pivot lines.
It is another object of the present invention to provide such a bearing with a tilting pad spring support system that assures retention of a reasonably hydrodynamically optimized scoop shape for the fluid foil converging ramps over a wide range of bearing operating speeds and bearing thrust loads.
It is another object of the present invention to provide such a bearing with foil elements that are not welded or brazed to form foil member assemblies.
It is another object of the present invention to provide such a bearing with pins (rigidly attached to the bearing housing) which position and resist rotation of the foil elements.
It is another object of the present invention to provide such a bearing with self shimming capability utilizing the resilient mounting and preload characteristics of the thrust disk, the spacer ring and the foil""s self shimming rings to prevent variations in bearing axial play and sway space due to variations in foil thickness and foil coating thickness.
It is another object of the present invention to provide such a bearing with fluid foil elements, spacer ring element, thrust disk element and thrust plate element that can be installed in the thrust bearing quickly and easily.